1. Field of the Invention
The present invention relates to a ceramic heater, and more particularly to a ceramic heater used, for example, for a ceramic glow plug and a method of manufacturing it.
2. Description of the Related Art
As a ceramic heater used, for example, for a ceramic glow plug, a structure has been known in which a resistive heating element made of a ceramic conductive material is embedded in an insulating ceramic substrate. As a conductive ceramic ingredient contained in the resistive heating element, silicide of a transient metallic element (e.g. molybdenum disilicide (MoSi.sub.2)) has been widely used in recent years because it has a swift heating property reaching a saturated temperature in a short time and can improve the starting property of an engine.
The ceramic heater as described above is commonly manufactured by preparing a composite mold in which a heating element mold is embedded in a mold of raw material powder of a ceramic element, the heating element mold being prepared by molding ceramic raw material powder containing conductive ceramic powder mixed with a prescribed quantity of organic binder into a shape of a prescribed resistive heating element, and sintering the composite mold at a prescribed temperature. If the conductive ceramic ingredient in the ceramic resistive heating element is MoSi.sub.2, it is important to adjust a sintering atmosphere as necessary so that the MoSi.sub.2 raw material powder does not deteriorate during sintering. For example, Examined Japanese Patent Publication 4-4715 discloses a technique for sintering the ceramic resistive heating element, composed of a matrix of Si.sub.3 N.sub.4 and a conductive ceramic component of MoSi.sub.2, in a non-oxidizing atmosphere not containing nitrogen (inclusive of the vacuum atmosphere) or a nitrogen-containing non-oxidizing atmosphere at a nitrogen partial pressure of 0.3 atm or lower. The publication describes the reason why the above sintering atmosphere is used: if a compound of Si.sub.3 N.sub.4 and MoSi.sub.2 is sintered at the nitrogen partial pressure of 0.3 atm, most of MoSi.sub.2 deteriorates and hence the temperature-rising characteristic of the ceramic resistive heating element is attenuated.
However, as a result of careful investigation by the inventors of the present invention, it has been found that as regards the ceramic resistive heating element containing transient metallic silicide, the deterioration of the metallic silicide cannot be restricted sufficiently by only adjusting the atmosphere during sintering so that the temperature-rising characteristic may be attenuated, or an electric resistance value may be altered, which makes it impossible to obtain an improved resistive heating element.